diff --git a/src/main/java/com/loomcom/symon/devices/Acia.java b/src/main/java/com/loomcom/symon/devices/Acia.java
index 12dc498..02fe0ea 100644
--- a/src/main/java/com/loomcom/symon/devices/Acia.java
+++ b/src/main/java/com/loomcom/symon/devices/Acia.java
@@ -33,7 +33,7 @@ import com.loomcom.symon.exceptions.MemoryRangeException;
 public abstract class Acia extends Device {
 
     private String name;
-    
+
     /**
      * Register addresses
      */
@@ -42,13 +42,14 @@ public abstract class Acia extends Device {
     boolean receiveIrqEnabled = false;
     boolean transmitIrqEnabled = false;
     boolean overrun = false;
-    
-	long lastTxWrite   = 0;
+    boolean interrupt = false;
+
+    long lastTxWrite   = 0;
     long lastRxRead    = 0;
     int  baudRate      = 0;
     long baudRateDelay = 0;
-	
-	/**
+
+    /**
      * Read/Write buffers
      */
     int rxChar = 0;
@@ -56,8 +57,8 @@ public abstract class Acia extends Device {
 
     boolean rxFull  = false;
     boolean txEmpty = true;
-	
-	
+
+
     public Acia(int address, int size, String name) throws MemoryRangeException {
         super(address, address + size - 1, name);
         this.name = name;
@@ -100,7 +101,7 @@ public abstract class Acia extends Device {
     /**
      * @return The contents of the status register.
      */
-    public abstract int statusReg();
+    public abstract int statusReg(boolean cpuAccess);
 
     @Override
     public String toString() {
@@ -117,13 +118,14 @@ public abstract class Acia extends Device {
     }
 
     public synchronized void rxWrite(int data) {
-        if(rxFull) {
+        if (rxFull) {
             overrun = true;
         }
-        
+
         rxFull = true;
 
         if (receiveIrqEnabled) {
+            interrupt = true;
             getBus().assertIrq();
         }
 
@@ -132,11 +134,12 @@ public abstract class Acia extends Device {
 
     public synchronized int txRead(boolean cpuAccess) {
         if (cpuAccess) {
-          txEmpty = true;
+            txEmpty = true;
 
-          if (transmitIrqEnabled) {
-            getBus().assertIrq();
-           }
+            if (transmitIrqEnabled) {
+                interrupt = true;
+                getBus().assertIrq();
+            }
         }
         return txChar;
     }
diff --git a/src/main/java/com/loomcom/symon/devices/Acia6551.java b/src/main/java/com/loomcom/symon/devices/Acia6551.java
index 9586b3c..b6c0b24 100644
--- a/src/main/java/com/loomcom/symon/devices/Acia6551.java
+++ b/src/main/java/com/loomcom/symon/devices/Acia6551.java
@@ -61,7 +61,7 @@ public class Acia6551 extends Acia {
             case DATA_REG:
                 return rxRead(cpuAccess);
             case STAT_REG:
-                return statusReg();
+                return statusReg(cpuAccess);
             case CMND_REG:
                 return commandRegister;
             case CTRL_REG:
@@ -178,8 +178,8 @@ public class Acia6551 extends Acia {
      * @return The contents of the status register.
      */
     @Override
-    public int statusReg() {
-        // TODO: Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
+    public int statusReg(boolean cpuAccess) {
+        // TODO: Parity Error, Framing Error, DTR, and DSR flags.
         int stat = 0;
         if (rxFull && System.nanoTime() >= (lastRxRead + baudRateDelay)) {
             stat |= 0x08;
@@ -190,6 +190,14 @@ public class Acia6551 extends Acia {
         if (overrun) {
             stat |= 0x04;
         }
+        if (interrupt) {
+            stat |= 0x80;
+        }
+
+        if (cpuAccess) {
+            interrupt = false;
+        }
+
         return stat;
     }
 
@@ -201,6 +209,7 @@ public class Acia6551 extends Acia {
         rxFull = false;
         receiveIrqEnabled = false;
         transmitIrqEnabled = false;
+        interrupt = false;
     }
 
 }
diff --git a/src/main/java/com/loomcom/symon/devices/Acia6850.java b/src/main/java/com/loomcom/symon/devices/Acia6850.java
index f904f94..bd70eed 100644
--- a/src/main/java/com/loomcom/symon/devices/Acia6850.java
+++ b/src/main/java/com/loomcom/symon/devices/Acia6850.java
@@ -43,7 +43,7 @@ public class Acia6850 extends Acia {
 
     static final int STAT_REG = 0;	// read-only
     static final int CTRL_REG = 0;	// write-only
-	
+
     static final int RX_REG = 1;	// read-only
     static final int TX_REG = 1;	// write-only
 
@@ -56,9 +56,12 @@ public class Acia6850 extends Acia {
     public int read(int address, boolean cpuAccess) throws MemoryAccessException {
         switch (address) {
             case RX_REG:
+                if (cpuAccess) {
+                    interrupt = false;
+                }
                 return rxRead(cpuAccess);
             case STAT_REG:
-                return statusReg();
+                return statusReg(cpuAccess);
 
             default:
                 throw new MemoryAccessException("No register.");
@@ -69,6 +72,9 @@ public class Acia6850 extends Acia {
     public void write(int address, int data) throws MemoryAccessException {
         switch (address) {
             case TX_REG:
+                if (cpuAccess) {
+                    interrupt = false;
+                }
                 txWrite(data);
                 break;
             case CTRL_REG:
@@ -78,7 +84,7 @@ public class Acia6850 extends Acia {
                 throw new MemoryAccessException("No register.");
         }
     }
-    
+
     private void setCommandRegister(int data) {
         // Bits 0 & 1 control the master reset
         if((data & 0x01) != 0 && (data & 0x02) != 0) {
@@ -97,8 +103,8 @@ public class Acia6850 extends Acia {
      * @return The contents of the status register.
      */
     @Override
-    public int statusReg() {
-        // TODO: Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
+    public int statusReg(boolean cpuAccess) {
+        // TODO: Parity Error, Framing Error, DTR, and DSR flags.
         int stat = 0;
         if (rxFull && System.nanoTime() >= (lastRxRead + baudRateDelay)) {
             stat |= 0x01;
@@ -109,7 +115,10 @@ public class Acia6850 extends Acia {
         if (overrun) {
             stat |= 0x20;
         }
-		
+        if (interrupt) {
+            stat |= 0x80;
+        }
+
         return stat;
     }
 
@@ -118,6 +127,7 @@ public class Acia6850 extends Acia {
         overrun = false;
         rxFull = false;
         txEmpty = true;
+        interrupt = false;
     }
 
 }